(1) Field of the Invention
This invention relates to a low-temperature curing coating composition. More particularly, the invention relates to a low-temperature curing amino-acrylic resin coating composition of a high solid type.
(2) Description of Prior Art
In the industrial fields of automobile manufacture, automobile repairing and other manufactures of industrial equipment, steel-made furniture, non-metallic products and so forth, several properties are required of the coating compositions to be used in view of the reduction of labour and energy and the improvement of safety to human body, in recent years.
For these purposes, acrylic lacquer, acrylic enamel, acryl-urethane resin paint and amino-alkyd resin paint have been hitherto used widely.
In connection with the acrylic lacquer, however, the coating must be done repeatedly for many times and the polishing with a polishing compound is required, so that the acrylic lacquer is not satisfactory in view of the reduction of labour and the environmental pollution caused by evaporation of a large amount of organic solvent. Also, the acrylic enamel is not satisfactory with respect to the film properties such as drying property, hardness, solvent resistance and the retention of gloss. Further, the acryl-urethane resin paint is undesirable in view of safety since it contains isocyanate compounds injurious to health. Furthermore, since the baking temperature of the aminoalkyd resin paint is high (for example, at 140.degree. to 160.degree. C. for 20 minutes), it cannot be regarded as satisfactory in view of the requirement of low-temperature curing (the term "low-temperature" herein referred to means the range of 60.degree. to 100.degree. C.).
In order to meet the requirements of the reduction of labour and energy and the improvement of safety, it is necessary to make the coating composition high-solid and low-temperature curable. Especially, the desirable coating composition must be cured at a temperature from 60.degree. to 100.degree. C., more preferably from 60.degree. to 80.degree. C. for 20-30 minutes, which is lower than the temperature commonly regarded as low temperatures.
In the case that a high-solid acrylic resin coating composition is prepared in the prior art, the molecular weight of the acrylic resin used is reduced by the using of chaintransfer agents or by increasing the use of polymerization catalyst. The low molecular weight acrylic resin thus obtained is employed to increase the solid content of a coating composition. (cf. for example, Journal of the Japan Society of Colour Material, Vol. 48, No. 11, pages 677-685, 1975) The acrylic resins can be classified according to their functional groups into methylolacrylamide type, hydroxy type, carboxyl type and glycidyl type. The hydroxy type is most widely used in combination with melamine resin and, when it is applied, the coating film is baked at 140.degree. to 150.degree. C. for about 30 minutes. However, since the molecular weight of this acrylic resin is low, the durability of obtained coating film is not satisfactory. Further, there is known a method to add an acid catalyst so as to lower the baking temperature of amino acrylic resin coating composition, however, when the properties of the obtained coating films are taken into consideration, the limit of baking condition is 100.degree. C. for 30 minutes.
The inventors of the present application have carried out wide and extensive studies in order to obtain a novel low-temperature curing and high-solid coating composition. As a result, the present invention has been accomplished through the following measures, without lowering the molecular weight of acrylic resin.
That is, (1) the glass transition temperature of acrylic copolymer is brought to 20.degree. C. to -50.degree. C. When the glass transition temperature is low, the solution viscosity of a resin can be made low, so that the preparation of a high-solid composition becomes possible. Further, the marvel is that the lower the glass transition temperature of an acrylic copolymer is, the higher the hardness of coating film becomes. This fact has been found out by us and the conditions for this fact are quite important for the coating composition of the present invention.
In the conventional coating compositions, the glass transition temperatures of acrylic copolymers were generally high in the range of 20.degree. C. to 105.degree. C., and it has been regarded that when a vehicle having a very low glass transition temperature as the acrylic copolymer of the present invention is used, the hardness of obtained coating film becomes low and the surface of coating film remains sticky. Therefore, the use of such vehicle has never been tried. From this point of view, the coating composition of the present invention is a novel one.
(2) It is necessary that the hydroxyl value of the above acrylic copolymer is in the range of 40 to 280.
The hydroxyl valve has influences on the hardness and the gasoline resistance. Especially when the glass transition temperature is low, the effect of hydroxyl value is quite large. Therefore, this hydroxyl value is also an important limiting factor for the coating composition of the present invention. Except the coating composition using isocyanate as a curing agent, there is not used acrylic resin having such a high hydroxyl value as that of the acrylic copolymer of the present invention. Much more, the coating composition of the present invention is quite novel in the compositions combined with aminoplasts.
(3) It is necessary that the alcohols used for the etherification of amino-formaldehyde resin are low carbon monohydric alcohols having 1 to 4 carbon atoms. This depends upon the fact that ether interchange reaction occurs mainly rather than deformaldehyde reaction at low temperatures and the aminoformaldehyde resin etherified with low carbon monohydric alcohols is subject to the ether interchange reaction.
(4) An acid catalyst is employed. By the use of the acid catalyst, the low temperature curing can be facilitated.
Further, in the field of non-metallic products, especially in the production of outdoor plywood, a durable and low-temperature curing coating composition has long been desired for and the coating composition of the present invention also meets such requirement forming a quite weather resistant and hard coating films.